A very large, rod-like protein related to spectrin and the alpha-actinins, expressed in many different tissues but most prominently in skeletal muscle. Dystrophin's primary function appears to be creating a structural link between the contractile machinery and the plasma membrane, or sarcolemma, of skeletal muscle cells, although there have been hints that it also performs signalling roles.

Mutations in the gene for dystrophin can have variable consequences. Mutations which result in small deletions of the middle part of the protein, leaving its two ends intact, often cause a mild, adult-onset muscle disease called Becker Muscular Dystrophy (BMD). Larger deletions, or mutations which prevent the complete synthesis of the protein, result in a severe and horrible muscle disease called Duchenne Muscular Dystrophy (DMD). DMD sufferers suffer from progressive wasting of their muscles, leaving them wheelchair-bound by early adolescence and resulting in death (usually from respiratory problems caused by atrophy of the diaphragm muscle) before their mid-twenties. Attempts to treat DMD, most recently using gene therapy, have thus far been almost entirely unsuccessful.